The present invention relates to an extruder for casting concrete slabs.
In a typical concrete slab extruder the concrete mix is dropped onto auger flights which force the concrete under pressure onto the casting bed. The bottom side of the concrete slab cross-section is defined by the form of the casting bed, the other sides being defined by the side and top mold plates of the extruder. The hollow channels or cavities in the slab are formed by the core members which follow the augers. A prior-art extruder with core members between the augers also exists.
The compacting of concrete is done with high-frequency vibrators. The vibration is then applied to the core members, the mold, the side mold plates, or the top mold plate, and in some cases to all of these. This extruder construction is widely used but has, e.g., the following disadvantages: The vibration compaction process generates heavy noise; the vibrating mechanism has a complicated construction and contains several wearing parts; and the concrete compaction is uneven between the thinner and thicker wall sections.
In addition, a further prior-art construction acting under the following principle exists.
In a first phase of the process, the extruder feeds a layer of concrete onto the casting bed. This forms the base section of the slab shell. In a next phase, another layer of concrete is fed between the tube-formed core members of the extruder. The core members perform a cyclic longitudinal movement to improve the homgenization of the concrete mix. In addition, the core members are vibrated at a high frequency to compact the concrete. The extruder then feeds a third layer of concrete over the core members, and finally a vibrating trowel beam performs the levelling and compaction of the upper surface.
Though the construction described above is widely used, it has, e.g., the following disadvantages: the concrete must be fed in several phases before the mold is sufficiently filled; the machine is not operable with a sufficiently low slump concrete mix; and the compacting vibration generates heavy noise.